Coal gasification with alkali additives to reduce emissions of mercury to the atmosphere

ABSTRACT

Method for removing mercury emissions from the burning of coal or other carbonaceous fuels, such as in a power plant or from coal gasification. Alkali additives are introduced in the coal gasification and staged coal combustion processes to capture the mercury in an alkaline molten slag. The combustor is operated at a stoichiometric air or oxygen to fuel ratio of about 0.40 to 0.80 and a temperature range of about 2200°-3000° F. During the staged combustion process the molten slag containing combinations of alkali and mercury is removed and disposed of to minimize or prevent mercury from escaping in the flue gas.

[0001] This Application claims priority from Provisional Patent SerialNo. 60/441,005 filed Jan. 17, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a method for the reduction of mercuryemissions from coal gasification processes. More particularly, it refersto an improved method for removal of mercury through the use of alkaliadditives in coal gasification and staged coal combustion processes.

[0004] 2. Description of the Prior Art

[0005] The 1990 Clean Air Act Amendments identified 189 substances thatwere designated as hazardous air pollutants (air toxins). Thesesubstances are chemicals, including heavy metals and organic compoundsin both solid and gaseous forms, known to pose a risk to human health.One metallic element, mercury, is getting much attention due to itsquantity and toxicity.

[0006] Mercury emissions to the air and releases to water occurnaturally and by human activities. According to a fairly recentemissions inventory (1994-1995), in the United States the major emittersof mercury to the atmosphere were electric utilities, municipal wastecombustors, commercial and industrial boilers, medical wasteincinerators, and chlor-alkali plants. Until the middle of the decade,municipal waste combustors, hazardous waste combustors, and medicalwaste incinerators were the leading emitting source category. The UnitedStates Environmental Protection Agency (hereinafter “EPA”) now regulatesthese industries, and the EPA estimates that emissions from municipalwaste combustors and medical waste incinerators declined by 90% from1990 to 2000. This currently makes coal-fired utilities the leadingman-made source of air-borne mercury emissions in the U.S. Of theestimated 5,000 tons of global mercury emissions emitted to theatmosphere in 1994-95, U.S. coal-fired power plants contributed about 51tons, or 1%. This rate of mercury emissions represented 33% of the 158tons of mercury released in the U.S. for the same time period.

[0007] There are several methods for removing elemental mercury and itscompounds from combustion/incineration flue gas. Elemental mercuryremoval is somewhat difficult because mercury remains in the vapor phaseat very low temperatures ( boiling point at 674° F.) and does notcondense on ash particles in the flue gas stream so that it may beremoved with electrostatic precipitators. However, removal of mercuryfrom combustion flue gas (U.S. Pat. No. 4,889,698 and U.S. Pat. No.5,672,323)) using activated carbon adsorption is known in the prior art.There are also other methods of removal; they include the use ofoxidizing agents that convert elemental mercury to its soluble compoundforms (U.S. Pat. No.5,900,042) so that it may be scrubbed from the fluegas. Another method, U.S. Pat. No. 6,214,304, uses alkali sulfides toconvert elemental mercury to mercury sulfide that is removed byparticulate control devices. Another method uses alkali injection intothe boiler furnace (U.S. Pat. No. 6,372,187); it has been shown to besomewhat effective in reducing mercury emissions. However, thesemethods, if highly effective (90% removal) like carbon adsorption arevery expensive techniques (as high as $100,000/lb of removal). Theoxidizing method (U.S. Pat. No. 5,900,042) and the alkali furnaceinjection method (U.S. Pat. No. 6,372,187) although less expensive, onlyremove 50 to 55% of the mercury.

[0008] It would therefore be advantageous to have an improved mercurycapture technique that will reduce coal mercury emissions to theatmosphere and do so at a relatively low cost. The method of the presentinvention is inexpensive and is as effective if not more effective thanthe carbon adsorption method.

SUMMARY OF THE INVENTION

[0009] I have discovered a process employing a staged combustor toremove mercury in an alkaline molten slag. High levels of mercurycapture were found to be an inherent feature of a staged combustor (seeU.S. Pat. Nos. 4,395,975, 4,423,702 and 5,458,659) developed for thereduction of sulfur and nitrogen oxides to the atmosphere. Alkalicompounds, such as limestone, lime, hydrated lime, dolomite, trona,nacholite or combinations thereof are added with the coal being fired inthe first stage of the combustor, or are added separately into the firststage of combustion operating at 2400 to 2700° F. The first stage ofcombustion, in effect, is a coal gasifier operating at an air to fuelstoichiometric ratio of around 0.60. Sulfur and high levels of mercurycapture are achieved through capture in the alkaline molten slagproduced from the partial oxidation of any carbonaceous fuel, includingcoal, by incorporating a combustor design that yields a reducingcondition in the alkaline molten slag sulfur capture zone. Nitrogenoxide emissions are also reduced by firing the coal in asubstoichiometric air condition in the first stage that reduces NO_(x)production from the oxidation of fuel bound nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein;

[0011]FIG. 1 shows a schematic of a staged combustion system applied toa coal-fired boiler; and

[0012]FIG. 2 shows the thermo chemical equilibrium for calcium andmagnesium oxide reactions with carbon to form elemental calcium andmagnesium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] It is believed that the reaction mechanism for mercury capture ina molten slag bath gasifier involves the formation and capture ofamalgams in complex mineral composites. Mercury will form amalgams withmany alkali metals, alkaline earth metals, zinc, cadmium (Ca), arsenic,antimony, gold, silver and copper. Other metals like molybdenum,manganese, cobalt and particularly iron are nearly insoluble in mercury.It is believed that the high melting point alkaline earth metals Ca(melting point of 2192° F.) and Magnesium (Mg) (melting point of 2030°F.) that are combined with their oxide forms CaO (melting point of 4658°F.) and MgO (melting point of 5072° F.) are the alkaline earth metalsthat are forming amalgams with mercury. Under reducing conditions withcarbon as the reducing agent for a gasifier temperature range of 2400 to2700° F., both elemental calcium and magnesium can form; see the thermochemical equilibrium coefficients for these reactions in FIG. 2.Although the equilibrium coefficients are low, still there would beorders of magnitude more concentration of elemental calcium andmagnesium to react with all of the mercury in the coal. Since the coalis fired into the alkaline molten slag bath with enough force to swirlthe slag, there should be plenty of carbon formed to create somequantity of elemental calcium and magnesium. Carbon monoxide will alsoreact with the oxides of calcium and magnesium to form elemental calciumand magnesium but the reactions are not quite as favored as thereactions with carbon.

[0014] To achieve high mercury capture, the combustor is designed toprovide for 1) intimate mixing of the carbonaceous fuel and itsreactants with the reduced alkaline molten slag, and 2) intimatefuel/air mixing, done in such a way as to eliminate the formation oflocalized pockets of unreacted oxygen. By keeping the molten slag in ahot reducing condition (2200 to 2700° F.), carbon and carbon monoxidereact with certain metals to convert a portion of those metals to theirelemental form that will then combine with mercury to form an amalgam;for example:

CaO_(solid)+C_(solid)→Ca_(solid)+CO_(gas)

CaO_(solid)+CO_(gas)→Ca_(solid)+CO_(2 gas)

[0015] Mercury (Hg) is easily converted from its oxide and sulfide(Cinnabar) forms to elemental mercury:

HgO_(solid)+C_(solid)→Hg_(vapor)+CO_(gas)

HgO_(solid)+CO_(gas)→Hg_(vapor)+CO_(2 gas)

HgS_(solid)+H_(2 gas)→Hg_(vapor)+H₂S_(gas)

[0016] For example, the elemental calcium then will react with elementalmercury to produce an amalgam that is tied up in a complex mineralcomposite.

Ca_(solid)+Hg_(vapor)→Ca_(o)Hg_(o solid amalgam)

[0017] The conclusion that amalgam formation is probably the cause ofthe nearly quantitative capture of mercury in the alkaline molten slagcomes from the work done by Sir Humphrey Davy. In the early 1800's, Davyattempted to decompose a mixture of lime and mercuric oxide by anelectric current and an amalgam of calcium was obtained. The separationof the mercury from the calcium was then so difficult that Davy was notsure if he had obtained pure metallic calcium. Electrolysis of lime andcalcium chloride in contact with mercury gave the same results.

[0018] Laboratory analysis for a three-stage combustor demonstration,wherein the first stage was operating at an air to fuel stoichiometricratio that ranged from 0.58 of 0.77, firing an Illinois #5 coal with3.39 wt % sulfur and with limestone being added at a Ca/S ratio of 0.85,showed the following results, see Table 1. TABLE 1 Mercury Capture Rate,Hg, Capture, Material: lb/hr ppmw Hg, lb/hr % Input: Coal 1669.7 0.0890.00014860 Limestone (as CaO/MgO) 96.5 0.030 0.00000289 Total 1766.20.00015149 Output: Slag 38.9 2.60 0.00010110 66.7 Fly Ash 156.5 0.260.00004069 26.9 Total 195.4 0.00014179 93.6

[0019] Although a stack test was not completed for mercury emissionsfrom the staged combustion system, from the weight rates and analyses ofthe different streams, mercury capture in primarily the first stage(gasifier) molten slag exceeded 90%. Even more impressive is that whenleaching procedure tests were completed on the first stage (gasifier)slag and the fly ash removed from the flue gas baghouse, there was noleaching of mercury. Both samples of leachate yielded 0.0000 mg/l ofmercury.

[0020] Mercury analyses were also completed on the ash from a coal-firedchain grate stoker at the same facility, firing the same Illinois #5coal. The mercury in the fly ash was 0.079 ppmw and the mercury in thegrate bottom ash was 0.01 ppmw. This shows that mercury capture using astoker is very low compared to the staged combustion system. This alsoindicates that for mercury capture to occur, a reducing condition mustexist and limestone or some other alkali must be added. Data taken froma slagging cyclone boiler operation, firing Illinois coal whereinalkalis were not added that was operating under an overall oxidizingcondition showed that about 8% of the mercury was captured in the bottomslag.

[0021] A typical example of the process of the present invention,preferably using the CAIRE™ staged combustor (U.S. Pat. Nos. 4,423,702and 5,458,659), is shown schematically in FIG. 1. Certain variationsfrom this schematic could be made with such variations still beingwithin the context of this invention. It will be understood by thoseskilled in the art that certain variations from this schematic could bemade with such variations still being within the context of the presentinvention. In the embodiment shown in FIG. 1, a first stage combustor 10is located in front of the entries 12 into the furnace 13. Openings 5into each of the combustors receive a conventional fuel such aspulverized coal 2, and an alkaline product such as lime or limestone 3with the carrier primary air 1 and the preheated air or oxygen 4.Alternatively, a coal water slurry pump could be used to conveypulverized coal to the combustor. Controlled partial oxidation of thecoal takes place in the combustor by regulation of the preheated (400°to 700° F.) secondary air or oxygen flow 4. The air (oxygen) to fuelstochiometric ratio (SR) in first stage combustor 10 is maintained atabout 0.40 to 0.70 (SR₁) through control of the preheated air or oxygenflow 4, and most for air preferably at about 0.60. With the first stagecombustor 10, the products of partial combustion in the form of a fuelgas and the molten slag from the ash portion of the coal plus theinorganic alkali compounds are separated in the first stage partialoxidation chamber 10, and a molten slag eutectic 7 containing alkalicompounds and coal ash exit through the bottom opening 8 of the firststage combustor 10. The molten slag is quenched in a water quench sluicesystem 9 and the ash is sluiced to a collection tank from where it ispumped to a settling pond, or otherwise disposed of according toconventional known methods.

[0022] The staged combustor 10 has a partial oxidation zone where mixingat a temperature of about 2200° to 3000° F. provides intimate contactbetween the coal and air or oxygen. Through the use of a stagedcombustor 10 that has incorporated molten slag removal, a highpercentage (75-90%) of the molten slag produced during partial oxidationof the coal is removed from the gas prior to entry into the furnace 14,and prior to further partial oxidation at entry 12.

[0023] Although certain embodiments of the invention have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alterations would be developed in light of theoverall teaching of the disclosure. Accordingly, the particularembodiments and arrangements disclosed herein are intended to beillustrative only and not limiting as to the scope of the inventionwhich should be awarded the full breadth of the following claims and inany and all equivalents thereof.

What is claimed is:
 1. A method for the removal of mercury ofcarbonaceous fuel comprising a) introducing any carbonaceous fuel; coal,coke, bio-mass or combinations thereof containing mercury into a firststage partial oxidation (gasifier) unit operating at a stoichiometricair or oxygen air to fuel ratio of 0.40 to 0 0.80, to provide a reducingoperating condition for high levels of mercury capture in an alkalinemolten fuel ash slag under reducing conditions with carbon, carbonmonoxide and hydrogen as the reducing agents for a partial oxidation(gasifier) temperature range of 2200° F. to 3000° F.; b) introducing analkali or any alkali or combinations thereof from the class consistingof lime, limestone, dolomite, calcium chloride, nacholite, and trona,with the said fuel or via a separate stream into the first stageoxidation unit, the alkali acting as a flux to reduce moltencarbonaceous fuel ash viscosity and to react with the mercury speciesbeing liberated from said fuel; c) fuel gas and molten slag beingseparated in a first stage cyclonic device following the fuel gas-slagmix section and said molten slag containing combinations of alkalis andmercury compounds being removed to a water quench system and disposedof.
 2. An apparatus for removing mercury during combustion of acarbonaceous fuel as shown in FIG. 1, and operated according to theparameters shown in FIG. 2.